as for poseidon, i use poseidon here and i dont need to turn that OFFLINE to use whdload, but miamidx must be offline.
Another problem with Amiga USB stuff is simply the Zorro card interrupts themselves. Just having my Highway card installed without starting Poseidon breaks many games in WHDLoad.
I'm not sure if this affects Z3 slots, Algor Cards, Subways, or anything else, but in my configuration (Highway card in a Z2 slot in the Z4 busboard) just having my Highway installed is causing interrupt problems.
I've been thinking about just connecting a bare A1200 or A500 to my TVCard's switchbox just to relive the old games once in a while. Which is a shame as WHDLoad worked so well before installing the Highway card - but there's no way I'll live without my Highway installed :-)